Field
Certain aspects of the present disclosure generally relate to magnetic tunneling junction (MTJ) devices, and more particularly to pinned layer structures of MTJ devices.
Background
Unlike conventional random access memory (RAM) chip technologies, in magnetic RAM (MRAM) data is stored by magnetization of storage elements. The basic structure of the storage elements consists of metallic ferromagnetic layers separated by a thin tunneling barrier. Typically, the ferromagnetic layers underneath the barrier (e.g., the pinned layer) have a magnetization that is fixed in a particular direction. The ferromagnetic magnetic layers above the tunneling barrier (e.g., the free layer) have a magnetization direction that may be altered to represent either a “1” or a “0.” For example, a “1” may be represented when the free layer magnetization is anti-parallel to the fixed layer magnetization. In addition, a “0” may be represented when the free layer magnetization is parallel to the fixed layer magnetization or vice versa. One such device having a fixed layer, a tunneling layer, and a free layer is a magnetic tunnel junction (MTJ). The electrical resistance of an MTJ depends on whether the free layer magnetization and fixed layer magnetization are parallel or anti-parallel to each other. A memory device such as MRAM is built from an array of individually addressable MTJs.
To write data in a conventional MRAM, a write current, which exceeds a critical switching current, is applied through an MTJ. Application of a write current that exceeds the critical switching current changes the magnetization direction of the free layer. When the write current flows in a first direction, the MTJ may be placed into or remain in a first state in which its free layer magnetization direction and fixed layer magnetization direction are aligned in a parallel orientation. When the write current flows in a second direction, opposite to the first direction, the MTJ may be placed into or remain in a second state in which its free layer magnetization and fixed layer magnetization are in an anti-parallel orientation.
To read data in a conventional MRAM, a read current may flow through the MTJ via the same current path used to write data in the MTJ. If the magnetizations of the MTJ's free layer and fixed layer are oriented parallel to each other, the MTJ presents a parallel resistance. The parallel resistance is different than a resistance (anti-parallel) the MTJ would present if the magnetizations of the free layer and the fixed layer were in an anti-parallel orientation. In a conventional MRAM, two distinct states are defined by these two different resistances of an MTJ in a bitcell of the MRAM. The two different resistances indicate whether a logic “0” or a logic “1” value is stored by the MTJ.
Spin transfer torque magnetic random access memory (STT-MRAM) is an emerging type of nonvolatile memory that operates at a higher speed than off chip dynamic random access memory (DRAM). In addition, STT-MRAM has a smaller chip size than embedded static random access memory (eSRAM), unlimited read/write endurance, and a low array leakage current.
In one category of STT-MRAM cells, a direction of polarization of the free layer and the reference layer of a magnetic tunnel junction (MTJ) is parallel to the plane of the respective layer. Such memory cells are referred to as having in-plane magnetic anisotropy or longitudinal magnetic anisotropy (LMA). The direction of magnetic polarization of MRAM cells with in-plane magnetic anisotropy is provided by constructing memory cells with an elongated shape, such as an ellipse. The elongated shape provides pole locations at each end for the magnetic moment to tend toward or away from.
In another category of STT-MRAM cells, the direction of polarization of the free layer and reference layer of an MTJ is perpendicular to the plane of the respective layer. Such memory cells are referred to as having perpendicular magnetic anisotropy (PMA). The direction of magnetic polarization in a layer of a PMA type of STT-MRAM cell is inherently directional, i.e., perpendicular to the layer. Therefore the PMA type STT-MRAM cells can have a symmetric shape, such as a circular shape, that can be more easily fabricated than elongated in-plane MRAM cells. Circular shaped PMA type STT-MRAM cells have a smaller area than elongated in-plane STT-MRAM cells and therefore may facilitate the development of more deeply scaled memory devices. Also, because the switching current of an STT-MRAM cell is proportional to its area, PMA type STT-MRAM cells may consume less power than in-plane type STT-MRAM cells. An MTJ in which the direction of polarization of the free layer and reference layer is perpendicular to the plane of the respective layer may be referred to as a perpendicular MTJ, or a pMTJ.